The present invention relates to a voltage regulator tube for an ignition system of an internal combustion engine, which is connected in series to a spark plug in the ignition system so as to be used.
An electric circuit of a known ignition system for an internal combustion engine is generally shown in FIG. 2. When a starter (not shown) is driven by closing a switch 10, a contact of a switch 12 is opened and closed by actuation of a cam (not shown) coupled with the starter. As a result, electric current flows through a primary winding of an ignition coil 11, so that a high voltage of several times ten KV is induced in a secondary winding of the ignition coil 11 and is supplied to a spark plug 14 through a distributor 13 and thus, sparks are generated between electrodes of the spark plug 14.
Meanwhile, such a phenomenon may happen in which the voltage required for spark discharge cannot be obtained due to carbon, lead compound, etc. adhering to surface of a porcelaneous member of the spark plug. In order to prevent such a phenomenon, it is known to provide a voltage regulator tube between the ignition coil and the spark plug. The assignee assigned by the present inventors proposed a voltage regulator tube of this kind in, for example, Japanese Patent Laid-Open Publication No. 2-94278 (1990).
In the voltage regulator tube of this prior art document, one of opposite openings of a hollow extending longitudinally through a cylindrical enclosure made of electrical insulating material is sealed, through sealing glass, by one metallic sealing member on which one main electrode is mounted. Likewise, the other opening of the hollow is sealed, through sealing glass, by the other metallic sealing member on which the other main electrode is mounted such that inert gas having a predetermined pressure is hermetically filled in the hollow. If this voltage regulator tube is connected between the spark plug and the secondary winding of the ignition coil in FIG. 2, adherence of carbon, lead compound, etc. to the surface of the porcelaneous member of the spark plug is lessened. However, the following problem still arises. Namely, when the voltage regulator tube is provided between the ignition coil and the spark plug, discharge starting voltage of the voltage regulator tube may drop or rarely rise under the influence of a grounding member disposed adjacent to the voltage regulator tube and thus, it becomes impossible to stably supply the predetermined high voltage to the spark plug.